Field of the Invention
The invention relates to a method of processing a signal in a hearing instrument, and to a hearing instrument, in particular a hearing aid.
The performance of the signal processing chain in a hearing instrument benefits from an adaptation to the acoustic environment. Examples for such adaptations are dereverberation and beamforming. Especially, dereverberation is an important challenge in signal processing in hearing instruments. Current technologies allow for only a crude estimate of the reverberation time for adaptation. There is a need to improve this.
Description of Related Art
According to a method of the prior art, dereverberation is achieved by convolving the reverberated signal with the inverse of the room impulse response. An early publication in this respect is Neely and Allen, J. Acoust. Soc. Amer. 66, July 1979, 165-169. The room impulse response is either assumed to be known or can be estimated from the audio signal to be reverberated. The latter case is usually referred to as blind deconvolution. Blind deconvolution and blind dereverberation is a field in which still a lot of research takes place.
U.S. Pat. No. 4,066,842 discloses a reverberation attenuation principle where the attenuation is given by the ratio of the cross-power spectral density and the sum of the two auto-power spectral densities. The types of microphones and their spacing are not specified. In an other publication, Allen et al. J. Acoust. Soc. Amer. 62(4), October 1997, the magnitude-square inter-aural coherence function is mentioned as an alternative, and this class of methods is now often referred to as coherence-based methods in literature. Bloom and Cain, IEEE Int. Conf. on ICASSP, May 1982, 184-187 have linked the pp coherence function to the direct-to-reverberant energy (DR) ratio but have failed to mention that the relationship is only correct for wavelengths smaller than the distance between the two microphones.
US 2005/244023 discloses a solution where the exponential decay due to reverberation in speech pauses is detected. Once the decay is detected, the spectrum is attenuated according to an estimate of the reverberant energy.
A method where blind source separation is combined with a coherence-based diffuseness indicator is disclosed in EP 1 509 065.
However, the methods according to the prior art suffer from substantial disadvantages. For dereverberation by deconvolution methods, the required room impulse response is generally not known in the hearing instrument context. Blind methods can currently only produce encouraging results for highly-idealized non-realistic scenarios. Their complexity is also far beyond what can currently be implemented in a hearing instrument. The methods that are based on detecting and attenuating the exponential decay are, in many situations, rather crude, and further improvements would be desirable. The coherence-based methods suffer from the fact that the distance between the two omni-directional microphones of a hearing instrument is so small that the pp-coherence is virtually identical to unity for direct and diffuse/reverberant sound fields. Better results are achieved when using the binaural coherence, but this requires a binaural link. Also, even then the diffuse/reverberant field coherence will have significant non-zero values for frequencies below about 600 Hz. Several experts in the field have now recognized that the coherence itself may not be the most appropriate parameter to control the spectral attenuation.